Effect of CPC micelle on N-hetero-aromatic base promoted room temperature permanganate oxidation of 2-butanol in aqueous medium

Article abstract of DOI:10.1016/j.molliq.2014.07.018

Oxidation kinetics of 2-butanol by permanganate ion under pseudo-first-order conditions, [2-butanol]_T [MnO₄ ^-]_T, in aqueous sulfuric acid solutions at 30 °C have been investigated spectrophotometrically. The pseudo-first-order rate constants and half-life were calculated from the ln(Absorbance)₅₂₅ versus time plot. The rates were found to be relatively slow in the uncatalyzed path, which increase by the presence of N-cetylpyridinium chloride (CPC) micelle and promoters: picolinic acid (PA) and 2,2-bipyridine (bipy). The catalytic effect of CPC on the permanganate oxidation of 2-butanol has been studied spectrophotometrically in the presence of promoter. CMC value of CPC was determined by three different techniques: conductometry, spectrophotometry and kinetics measurements. NMR and FTIR spectra confirmed the oxidized products. The aggregation and morphological changes during reaction were studied by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). The variation of the reaction rates for the different promoters in the presence and absence of CPC micellar catalyst is discussed qualitatively in the terms of partitioning nature of surfactant, charge of surfactant and reactants. Bipy in association with CPC micellar catalyst exhibited ~ 100-fold rate enhancements compared to the uncatalyzed reaction path.

Full text of DOI:10.1016/j.molliq.2014.07.018

Journal of Molecular Liquids 198 (2014) 369–380
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Effect of CPC micelle on N-hetero-aromatic base promoted room
temperature permanganate oxidation of 2-butanol in aqueous medium
b
Aniruddha Ghosh a, , Kriti Sengupta , Rumpa Saha a,c, Bidyut Saha
⁎
a,⁎⁎
a
Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan, Golapbag, Burdwan, 713104 WB, India
Department of Microbiology, The University of Burdwan, Golapbag, Burdwan, 713104 WB, India
K. G. Engineering Institute, Bishnupur, Bankura, WB, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxidation kinetics of 2-butanol by permanganate ion under pseudo-first-order conditions, [2-butanol]T
≫
[MnO−4 ]T, in aqueous sulfuric acid solutions at 30 °C have been investigated spectrophotometrically. The
pseudo-first-order rate constants and half-life were calculated from the ln(Absorbance)525 versus time plot.
The rates were found to be relatively slow in the uncatalyzed path, which increase by the presence of
N-cetylpyridinium chloride (CPC) micelle and promoters: picolinic acid (PA) and 2,2-bipyridine (bipy). The cat-
alytic effect of CPC on the permanganate oxidation of 2-butanol has been studied spectrophotometrically in the
presence of promoter. CMC value of CPC was determined by three different techniques: conductometry, spectro-
photometry and kinetics measurements. NMR and FTIR spectra confirmed the oxidized products. The aggrega-
tion and morphological changes during reaction were studied by Scanning Electron Microscope (SEM) and
Transmission Electron Microscope (TEM). The variation of the reaction rates for the different promoters in the
presence and absence of CPC micellar catalyst is discussed qualitatively in the terms of partitioning nature of sur-
factant, charge of surfactant and reactants. Bipy in association with CPC micellar catalyst exhibited ~100-fold rate
enhancements compared to the uncatalyzed reaction path.
Received 23 April 2014
Received in revised form 9 June 2014
Accepted 15 July 2014
Available online 28 July 2014
Keywords:
2-Butanol
Permanganate
N-cetylpyridinium chloride (CPC)
2,2-Bipyridine (bipy)
Spectrophotometry
Promoters
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
importance in green chemistry. Permanganate ion is a strong oxidizing
agent which still remains as one of the most versatile and vigorous ox-
Oxidation of 2-butanol to 2-butanone, commonly known as methyl
ethyl ketone (MEK) is an important transformation in synthetic chemis-
try. In particular, 2-butanol is used as an intermediate for devulcanizing
cured rubber and for the production of alkyl esters for use as a biodiesel
fuel. 2-Butanone is an effective and common solvent and is used in pro-
cesses involving gums, resins, cellulose acetate and nitrocellulose coatings
and in vinyl films, plastics, textiles, in the production of paraffin wax, and
in household products such as lacquer, varnishes, paint remover, a dena-
turing agent for denatured alcohol, glues, and as a cleaning agent [1,2].
Chromate and permanganate ions in various forms are used as pow-
erful oxidizing agents in organic and inorganic oxidations in polar
media [3]. The contamination of aquatic environment by toxic metals,
such as hexavalent chromium is of great concern due to its trends to ac-
cumulate on vital organs of human and animals causing several health
problems [4,5]. The permanganate oxidation process is eco-friendly
compared to other oxidants such as chromates [6–9] and has gained
idant used for oxidation of organic and inorganic compounds in acidic
media [10,11]. Permanganate, an important oxidant in many organic
and inorganic redox reactions, involves the Mn(VII) entity. In acid solu-
tion, permanganate is reduced to Mn2+ by an excess of reducing agent
(E0 = 1.51 V): MnO4− + 8H+ + 5e− → Mn2+ + 4H2O. The highest
oxidation state of manganese corresponds to the total number of 3d and
4 s electrons. The mechanisms for different organic substrates suggested
by various authors are not similar, indicating that a variety of mechanisms
are possible, depending upon the nature of the reactive manganese spe-
cies, the reaction environment and the nature of the substrate [10–12].
The ‘green chemistry’ has encouraged many synthetic organic chem-
ists to use water as the only solvent, because 80% of chemical waste is es-
timated to be organic solvents. The alternative is to use amphiphiles that
formed micelles to accommodate lipophilic environment for the insoluble
reactants or reagents. Reaction rates might be faster than those normally
observed in organic media, because the effective concentration within the
micelle could be quite high. This concept is called micellar catalysis and
many researches in this field have been reported [13,14].
⁎
Corresponding author.
Micellar properties have attracted growing attention for use in
biochemistry, biological, industrial and chemical research applications.
Since the surfactant is providing a very limited amount of an organic
medium in which these reactions can take place, reactions occur
⁎⁎ Corresponding author. Tel.: +91 9476341691 (mobile), +91 342 2533913 (O); fax:
+91 342 2530452 (O).
E-mail addresses: aniruddhachem27@gmail.com (A. Ghosh), b_saha31@rediffmail.com
(B. Saha).
http://dx.doi.org/10.1016/j.molliq.2014.07.018
0167-7322/© 2014 Elsevier B.V. All rights reserved.

370
A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
under typically high internal concentrations and under mild, usually
room temperature conditions [15]. The salient properties of the surfac-
tants that affect electron transfer reactions are localization and com-
partmentalization effect, pre-orientational polarity and counter-ion
effect and the effect of charged interfaces [16–19]. Mechanistic studies
on permanganate oxidation reaction of both organic and inorganic
substrates are invariably encumbered by difficulties due to multitude
of possible oxidation states [20]. Pyridinium surfactants are often uti-
lized as corrosion inhibitors, as well as being used in emulsion polymer-
ization, the flotation of minerals, and textile processing. Biological
applications of these surface-active agents include their antimicrobial
activity, as well as their use as drug and gene delivery agents [21].
Cetylpyridinium chloride (CPC) belongs to a class of frequently used
cationic surfactants having wide ranging applications, as drug delivery
vehicles, for catalysis, as nanoreactors, etc., it is also being used as an an-
tibacterial (antiseptic) drug. It is an active ingredient in mouthwashes
[22].
centrifuged to get a complete mixing through Centrifuge-Z206A
(Hermle Labortechnik GmbH).
2.2. Kinetic measurements
Reaction mixtures containing the known quantities of the substrate
(2-butanol), and acid under the kinetic conditions, [2-butanol]T
≫
10[MnO−4 ]T were thermostated at 30 °C ( 0.1 °C). A mixture contain-
ing the required amount of permanganate, CPC, water and other re-
agents (whenever necessary) was thermally equilibrated at a desired
temperature and to this was added a measured amount of 2-butanol
solution. The absorption measurements were made in a thermostated
cell compartment at the desired temperature within (30 0.1 °C) on a
UV–VIS–NIR Spectrophotometer (SHIMADZU-3600) automatic scanning
spectrophotometer fitted with a Temperature Control System program
controller using cells of path length 1 cm. The rate of disappearance of
the permanganate ion was monitored at 525 nm (λmax of MnO4−
)
Micellar catalysis oxidation of 2-butanol by CPC in aqueous acidic
solutions is usually explained on the basis of a distribution of reactants
between water and the micellar “pseudophase”. If the solubility of 2-
butanol is usually greater in the CPC micelles than in water the local
concentration is increased in the micelle, often with suitable orientation
of the reactants bound in the micelle. This leads to a large increase in the
effective concentration leading to an increase in the reaction rate.
Comparison to the organic solvents as reaction media modern micellar
catalysis involves small amounts of environmentally benign designer
surfactants [23,24].
The details of mechanism and oxidation kinetics of permanganate
oxidation of 2-butanol are also not yet known in the presence of surface
active N-cetylpyridinium chloride (CPC) and N-hetero-aromatic chelat-
ing ligands: picolinic acid (PA), 2,2′-bipyridine (bipy). PA and 2,2′-
bipyridine are N-hetero-aromatic chelating agents and form complexes
with chromium, zinc, manganese, copper, iron and molybdenum using
nitrogen and oxygen atom [25–29]. Furthermore, due to the presence
of low-energy π* orbitals of the ligand, metal complexes can be charac-
terized by strong metal-to-ligand charge-transfer (MLCT) absorption
bands in the visible spectrum [30]. In this paper we have for the first
time reported the incorporation of permanganate ion into the stern
layer of CPC micelles. Mainly, the oxidations are more efficient and se-
lective, the reaction conditions are milder, and the workups are easier.
On the other hand, owing to the reaction under aqueous micellar medi-
um, combustion, toxicity, and environmental pollution of solvents are
reduced.
[10,11]. It was verified that there is no interference from other reagents
at this wavelength. The reaction was started by adding the requisite,
and thermally equilibrated, solution of 2-butanol. The pseudo-first-
order rate constants (kobs; s−1) were obtained from the measurements
of slopes of the plots (Fig. S1, supplementary file) of log(absorbance) ver-
sus time. All experiments were studied in duplicate and the rate con-
stants were found to be reproducible within 3%. The kinetics of the
fast reactions was studied by using an Applied Photophysics SX-20
(Kinet Assist) stopped-flow spectrophotometer. Values of the rate con-
stants for the 2-butanol-MnO−4 reactions determined from the slopes
of the appropriate plots are presented in Table 1.
2.3. Product analysis
The oxidation product of 2-butanol, was qualitatively analyzed. After
ensuring completion of the reaction, the oxidized product was separat-
ed by fractional distillation of the reaction mixture. The corresponding
oxidized product (carbonyl compound) was efficiently separated by
fractional distillation [16,25]. The 1H NMR spectra of the product
carbonyl compound in CDCl3 solvent were obtained on a NMR spectro-
photometer (400 MHz, Bruker Ascend) operating at 400 MHz frequency
(Fig. S2, supplementary file). In this study the 2-butanone was the main
product of oxidation of 2-butanol. Qualitative identifications of the car-
bonyl products of MnO−4 oxidation reactions were made by the forma-
tion of yellow or yellow orange colored crystals of 2,4-dinitrophenyl
hydrazone derivative precipitated directly by the addition of 2,4-
dinitrophenyl hydrazene in the reaction mixtures [32]. The hydrazone
precipitate was filtered off and was recrystallized to determine the
melting points. The melting points 141–142 °C were matched with the
earlier reports [33,34]. The crystalline 2,4-DNP derivatives were
thoroughly mixed with KBr, pressed into a form of disk (pellet), to re-
cord FTIR spectrum by using FTIR spectrophotometer (IR Prestige 21,
SHIMADZU) and compared with spectra of the derivatives of known ke-
tone (Fig. S3, supplementary file). Thus, we may safely conclude that
carbonyl compound is the main oxidation product.
2. Experimental
2.1. Materials
2-Butanol (99%, Merck, India), potassium permanganate
(99%, Merck, India), sulphuric acid (99%, Merck, India), and CPC
(99%, Merck, India) were used. Permanganate solutions were stored in
a dark glass bottle and standardized by titration against oxalate. The sol-
vent used was water, previously doubly distilled, deionized and CO2
free. It was standardized against oxalic acid by following the literature
method [31]. PA (AR, Sigma Aldrich), 2, 2′-bipyridine (AR, E Merck),
N-cetylpyridinium chloride (AR, SRL), Sulphuric acid (AR, Merck), and
all other necessary chemicals used were of highest purity available com-
mercially. Solutions were prepared by using doubly-distilled water.
Doubly-distilled water was used throughout the work. Stock solution
of 2-butanol was prepared by dissolving the appropriate amount of
the sample in doubly-distilled water. The solution of CPC was prepared
by using the weighing balance (Sartorius BSA224S-CW) and dissolving
calculated amount of CPC in doubly-distilled water through Digital
Ultrasonic Cleaner CD 4820 instrument. Surfactant substrate mixture
solution and other mixture solution containing surfactant were
2.4. Stoichiometry
Stoichiometry of the reaction was determined by analytical method.
In this method the reaction mixture containing excess of KMnO4
over the 2-butanol was allowed to stand at 30 °C for a sufficiently
long time. The amount of unreacted permanganate was estimated
iodometrically. It has been found that the five molecules of 2-butanol
recognize two molecules of permanganate for complete oxidation. On
the basis of literature available [35,36] and the work performed, the
reaction proceeds as follows:
5 CH3CH2CHðOHÞCH3 þ 2MnO−4 þ 6Hþ→5CH3CH2COCH3 þ 2Mn2þ þ 8H2O

A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
371
Table 1
Effect of promoters and CPC micelle on pseudo-first-order rate constant and half-life* for the MnO−4 oxidation of 2-butanol.
103 × Substrate
104 × MnO−4
103 × promoter
103 × Micellar
catalyst
105 × kobs (s−1
)
Half life
(t1/2) (h)
(mol dm−3
)
(mol dm−3
)
(mol dm−3
)
(mol dm−3
)
2-butanol
1.0
2.0
3.0
4.0
5.0
1.0
1.0
–
–
2.75
6.816
7.883
10.166
11.12
2.75
2.633
2.36
2.416
2.52
4.98
5.716
6.56
8.56
8.766
5.85
7.133
9.55
10.1
13.783
98.3
104.6
118.3
124.3
131.0
135.0
140.0
253.8
281.0
0.02
0.04
0.06
0.05
0.01
0.01
0.03
0.01
0.05
0.04
0.02
0.06
0.03
0.04
0.02
0.02
0.01
0.01
0.04
0.02
0.04
0.04
0.05
0.06
0.05
0.01
0.02
0.04
0.06
7.0
2.82
2.44
1.89
1.73
7.0
2-butanol
2-butanol
2-butanol
2-butanol
1.0
2.0
3.0
4.0
5.0
1.0
–
–
7.31
8.16
7.97
7.64
3.86
3.37
2.93
2.25
2.20
3.29
2.70
2.01
1.90
1.40
0.19
0.184
0.16
0.155
0.147
0.143
0.138
0.086
0.0685
1.0
1.0
1.0
PA
bipy
–
1.0
2.0
3.0
4.0
5.0
1.0
2.0
3.0
4.0
5.0
–
1.0
1.0
–
CPC
0.3
0.6
0.9
1.2
1.5
1.8
2.1
1.2
1.2
2-butanol
2-butanol
1.0
1.0
1.0
1.0
PA
bipy
CPC
CPC
[H2SO4]T = 1.0 mol dm−3, Temp = 30 °C.
*The t1/2 values are directly calculated in this table by using the relation t1/2 = (ln2/kobs), where ln2 = 0.693, kobs = pseudo-first-order rate constant in s−1
.
2.5. Polymerization test
3. Results
3.1. UV–vis spectrophotometric study of the reaction
Acrylonitrile [40%] was added to the reaction mixture which was
allowed to stand for 2–3 h. No polymer formation was detected. The
result indicates the absence of any free radical intermediate and thus
the possibility of a one-electron transfer step may be ruled out [37].
The result is in conformity with a one-step two-electron transfer pro-
cess with no free-radical intermediate.
The MnO−4 ion, is a facile oxidant of organic compounds such as alco-
hol. In that case, 2-butanol, was oxidized into 2-butanone. Due to the
excess concentration of alcohol present in the solution as compared to
MnO−4 , it is expected that the Mn(VII) ion will be totally reduced to
Mn(II) at the end of the reaction. The color of the reaction mixture
changed from purple to colorless at the end of the reaction. Solutions
of Mn(VII) salts were purple in the presence of alcohol, whereas the
solutions of Mn(II) salts are colorless. The progress of the oxidation of
alcohols with MnO−4 in the presence and absence of surfactant and pro-
moters was reflected by marked changes in the electronic spectrum of
certain time intervals (Figs. S5–7, supplementary file). The scanned ab-
sorption spectra of the different sets of reaction mixtures were taken for
both in the presence and absence of surfactant and promoter. Replicate
scans of the spectra during the course of the reaction showed a decrease
in the absorbance only, with no evidence of any shift in the peaks
(Figs. S5–7, supplementary file). Spectroscopic test was carried out by
comparing the electronic spectrum of the reaction mixture 1 min after
the start of the reaction with that of the final product within a wave-
length of 250–650 nm.
2.6. Critical micelle concentration (CMC) determination
The CMC of CPC in the presence and absence of permanganate
and 2-butanol was determined at break points of nearly two straight-
line portions of the specific conductivity versus concentration
plot [20]. The CMC value of CPC was found to be 1.0 × 10−3 mol dm−3
by conductometry in the presence of only 2-butanol (1.0 ×
10−3 mol dm−3) (Fig. S4a, supplementary file). The same CMC
value was found by the absorbance measurement in UV in the presence
of permanganate (1.0 × 10−4 mol dm−3) (Fig. S4b, supplementary file).
The small change in the CMC could be associated to the formation of
ion-pair between permanganate ion and surfactant molecules through
electrostatic interactions. To determine the CMC, the conductivity mea-
surements of the surfactant (CPC) solutions were made with conductiv-
ity meter (EUTECH Instruments CON 6000, cell constant 1 cm−1), the
CMC values of these CPC was determined from plots of the specific con-
ductivity versus [surfactant] in the presence of 2-butanol. The break
points at the curve were taken as an indication of micelle formation
and this corresponds to the CMC of CPC [38]. The experiments were
carried out at 30 °C under varying conditions, that is, water + CPC +
2-butanol (conductometry), water + CPC + 2-butanol + MnO4−
(spectrophotometry).
3.2. Dependence of reaction rate on [MnO−4 ]
The influence of [MnO4−
] was studied at fixed [2-butanol]
(=1.0 × 10−3 mol dm−3), [H2SO4] (=1.0 mol dm−3), at different
[MnO4−] (1.0 × 10−4–5.0 × 10−4 mol dm−3), at temperature
(=30 °C). The kinetic curves were nonlinear showing an initial slow
stage (noncatalytic) followed by a fast stage (autocatalytic). It was ob-
served that rate constants of noncatalytic path decrease with increasing

372
A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
[MnO−4 ]. These results are against the norms of the kinetics (the
pseudo-first-order rate constants should be independent of the initial
concentration of the reactant in defect). It may be possible due to the
formation and flocculation (coagulation) of colloidal particles of
MnO2 during the course of reaction [20,39]. The independence of
pseudo-first-order rate constants obtained from the linear portions of
-ln(absorbance)-time plots (Fig. S1, supplementary file) at various
[MnO−4 ], may confirm that the reaction is first-order with respect to
permanganate ion concentration [40]. The first-order dependence
with respect to [MnO−4 ] was confirmed not only from the linearity of
the pseudo-first-order plots but also from the independence of the reac-
tion rate (Table 1) on various initial concentrations of permanganate
ranging between 1.0 × 10−4 and 5.0 × 10−4 mol dm−3 [41]. The values
of kobs for 2-butanol were found to be almost constant (Table 1) and in-
dependent of the initial [MnO−4 ] indicating that the reaction is first
order with respect to [MnO−4 ] [37].
3.3. Dependence of reaction rates on [2-butanol]
Fig. 2. Plot showing the effects [CPC] on kobs of the oxidation of 2-butanol by
permanganate. Reaction conditions: [MnO−4 T = 1.0 × 10−4 mol dm−3, [2-butanol]T
] =
1.0 × 10−3 mol dm−3, [H2SO4]T = 1.0 mol dm−3, T = 30 °C, the break point of the
curve is the CMC (0.9 mM) of CPC.
The reaction was studied at different concentrations of 2-butanol
without the addition of surfactant within the range 1.0 × 10−3 to
5.0 × 10−3 mol dm−3 concentrations. The plot of velocity constant
“kobs” against 2-butanol concentration is found linear (Fig. 1) showing
the first order dependency of the rate on 2-butanol concentration in
the given range for both stages. It has been observed that the rate of
reaction increases with substrate concentration. Velocity constants cal-
culated by different methods are given in Table 1. Under pseudo-first-
order conditions, viz. [2-butanol] ≫ [MnO−4 ], the reaction rate increases
with increasing [2-butanol]. The order in [2-butanol] was found to be
unity. This can be readily seen from the plot of kobs versus [2-butanol].
of 2-butanol and 1.0 × 10−4 mol dm−3 of MnO−4 . In our experiment,
the kinetic studies were limited in the [CPC] range of 3.0 × 10−4 to
21.0 × 10−4 mol dm−3. The plot of kobs against [CPC] shows gradual in-
crease of rates with the increase in [CPC] (Fig. 2) which clearly demon-
strate the CPC catalytic effect not only above but even below CMC,
i.e., micellar as well as submicellar catalysis are observed. These obser-
vations are most interesting instead of kobs, attained constant values
(for unimolecular reactions) or pass through a maximum (for bimolec-
ular reactions) with [CPC]. The observed catalytic effect may, therefore,
be due to (i) presence of premicelles and (ii) preponement of micelliza-
tion by reactants. The values of rate constants are summarized in
Table 1. It is certainly possible that the negative charge on MnO4−
3.4. Dependence of reaction rate on [CPC]
The effect of [CPC] (=3.0 × 10−4 to 18.0 × 10−4 mol dm−3) was
studied at constant [MnO−4 ] (=1.0 × 10−4 mol dm−3), [2-butanol]
(=1.0 × 10−3 mol dm−3), [H2SO4] (=1.84 × 10−4 mol dm−3) and
temperature (=30 °C). The kobs–[CPC] profile shows increase in the
rate constants with [CPC] that reaches a limiting value (Fig. 2). It has
been observed that the rate of reaction increases with surfactant con-
centration showing catalytic function of surfactant in the present
reaction.
forms an ion pair (Py+MnO4−) with the positive pyridinium (Py+
;
–PyN+(C16H33)) head group of CPC molecules which brings the reac-
tants together through electrostatic interactions [42]. Coordination of
a Py+ cation by the permanganate anion would decrease electron den-
sity of the MnO−4 which, in turn, increases the oxidizing power of the
permanganate. Fig. 2 signifies that MnO−4 and 2-butanol are interact
with the CPC surfactant, and submicellar aggregates are formed [38].
Both the reactants will be preferentially located at the positively
charged CPC aggregated molecules and, therefore, the kinetic CMC of
CPC is lower than in water. During the oxidation of 2-butanol by per-
manganate in the presence of CPC micellar catalyst the CMC value
obtained for CPC was 0.9 mM by kinetic methods.
3.4.1. Rate constants—[CPC] profile for oxidation of 2-butanol by MnO4−
In order to see the role of surfactants on the oxidation of 2-butanol
by MnO−4 , the effects of [CPC] were studied at 1.0 × 10−3 mol dm−3
3.5. Dependence of reaction rate on added promoters
3.5.1. Effect of PA
The concentration of PA was varied in the range 1.0–5.0 ×
10−3 mol dm−3 keeping all other conditions constant. It was observed
that the rate constant was increased with increasing concentration of
PA (Table 1, Fig. 3a).
3.5.2. Effect of 2,2′-bipyridine
The concentration of 2,2′-bipyridine was varied in the range
1.0–5.0 × 10−3 mol dm−3 keeping all other conditions constant. It
was observed that the rate constant was increased with increasing con-
centration of bipy (Table 1, Fig. 3b).
3.5.3. The UV–visible spectra of N-hetero-aromatic base promoters
The electronic spectra of the ligands and there complexes were
recorded in water and are given in Fig. 4a. The free ligands picolinic
Fig. 1. Dependence of kobs on [2-butanol] for MnO−4 oxidation of 2-butanol in aqueous
H2SO4 media at 300: [MnO4− T = 2 × 10−4 mol dm−3, [2-butanol]T = 2 × 10−3 mol dm−3
] .

A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
373
acidic, alkaline and neutral media. The reactivity of these intermediate
states depends on the nature of the reductant and on the pH of the me-
dium. Scheme 1 represents protonation of MnO−4 . The next reaction
shows the formation of a 2-butanol–Mn(VII) complex. We may assume
that the intermediate decomposes in a one-step, two-electron oxida-
tion–reduction mechanism [45] to Mn(V) and other oxidation products.
Mn(V) is highly unstable in an acidic medium with respect to dispro-
portionation and immediately gets converted into Mn(IV). An addition
to an O\H bond results in the formation of an ester; an addition to a
C\H bond gives an organometallic compound. The reducing behavior
of methanol and ethanol toward permanganate in a perchloric acid me-
dium has been investigated in the absence and presence of the surfac-
tant Tween-20 [46]. Permanganic acid is the chief oxidizing species,
which combines with the alkanol to form an intermediate complex
followed by decomposition in the rate-determining step to produce
the corresponding aldehyde and Mn(V). The latter disproportionates
to Mn(VII) and Mn(II) in the fast step (Scheme 1) [12].
The decomposition of the intermediate complex RCH2O.MnO3 may
be thought to occur as follows:
In the case where permanganate serves as an oxidizing agent in an
acid medium, the possible intermediate species are Mn(VI), Mn(V),
Mn(III) and Mn(IV). On the other hand, Mn(II), the ultimate reaction
product, acts as an autocatalyst. The plot of kobs versus [2-butanol] is
linear (Fig. 1), therefore, first-order with respect to [2-butanol]. The for-
mation of a single isosbestic point in the CPC catalyzed path near λ =
420 nm confirms the formation of Mn(IV) intermediate during the
reaction and as soon as it forms it readily decays to lower valent
manganese species. Under the experimental conditions (Figs. S5–7, sup-
plementary file) no manganese(IV) formation was observed in the pres-
ence of [H+] ≥ 1.0 mol dm−3. These results are not surprising because
Mn(IV) is known to be unstable in acidic medium. Again the presence
of Mn(VI) and Mn(V) is ruled out by the fact that they are highly unsta-
ble in acidic medium [47]
Fig. 3. Dependence of kobs on [promoter] for MnO−4 oxidation of 2-butanol (in absence of
surfactant) in aqueous H2SO4 media at 30 °C where [MnO−4 T = 2.0 × 10−4 mol dm−3
] ,
[2-butanol]T = 2.0 × 10−3 mol dm−3, [H2SO4]T = 1.0 mol dm−3. (a) Dependence of
kobs on [PA]; (b) Dependence of kobs on [bipy].
4.1.1. Rate law
Since the reaction gives a linear plot, it is proposed that the reaction
involves a pre-equilibrium with the formation of an intermediate com-
plex between HMnO4 and the 2-butanol molecule followed by the de-
composition of the complex in the rate limiting step to produce the
corresponding carbonyl compound and Mn(V). The latter undergoes dis-
proportionation to Mn(VII) and Mn(II) in a fast step (Scheme 1). The ex-
istence of Mn(V) as an intermediate species has already been reported
by a number of workers [48]. Disproportionation of manganese(V) in a
fast step produces manganese(VII). According to the suggested mecha-
nism, the rate of disappearance of MnO−4 may be expressed as:
acid and 2,2′-bipyridine UV–vis spectral data display two bands
~220–230 nm and ~270–280 nm attributed to π → π* and n → π* tran-
sitions. The electronic spectrum of the Mn(II) complex exhibited three
bands in the region near 210–280 nm which may be assigned to the
transitions 6 1g → 4T1g (4G), 6A1g 4T2g (4G) and 6A1g 4Eg, 4T1g
A → →
(4P) respectively, indicating octahedral geometry. The high-energy
band 220 nm is due to forbidden ligand → metal charge transfer [43].
Attempts to obtain spectral (UV–visible) evidence for the promoter-
permanganate complex was done and presented in Fig. 4b. The UV–
vis spectrum of Mn(II) solution shows intense bands at λmax = 215
and 290. The UV–vis spectra of Mn(II)-PA and Mn(II)-bipy complex
are shown in Fig. 4c which shows the spectra has two bands near
200–230 nm are attributed to the π–π* transition of the benzene ring
of PA and bipyridine. The n-π* transition of the C_N bond, respectively,
while the next band near 230–330 nm, as the most characteristic ad-
sorption band of Mn(II)-promoter complex is due to the MLCT [44].
Â
Ã
kdK1K2 Hþ ½CH3CH3CHðOHÞCH3ꢀ½MnO−4 ꢀ
1 þ K2½CH3CH2CHðOHÞCH3ꢀ
d½MnO−4 ꢀ
−
¼
dt
so that the pseudo-first-order rate constant
Â
Ã
kdK1K2 Hþ ½CH3CH3CHðOHÞCH3ꢀ
1 þ K2½CH3CH2CHðOHÞCH3ꢀ
1
d½MnO−4 ꢀ
Â
Ã
kobs¼ −
ꢁ
¼
MnO−4
dt
4. Discussion
The stable oxidation product of MnO−4 in acid medium is Mn(II). Fur-
thermore, because no rise and fall in the absorption was observed at
418 nm, it is concluded that Mn(IV) does not intervene as a possible ox-
idizing agent since it is short-lived. Mn2+ was identified by UV–visible
spectroscopy (Fig. 4c) and spot test [48]. The unstable intermediate
4.1. Reaction mechanism of uncatalyzed path
During the oxidation, it is evident that permanganate is reduced to
various oxidation states, i.e., Mn(VI), Mn(V), Mn(IV) and Mn(III) in

374
A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
Fig. 4. (a) Absorption spectrum of only promoter solution in aqueous medium: (A) Absorption spectrum picolinic acid, [PA]T = 1.0 × 10−4 mol dm−3; (B) Absorption spectrum
2,2′-bipyridine [bipy]T = 1.0 × 10−4 mol dm−3, Temp = 30 °C. (b) Absorption spectrum of reaction mixture with and without promoter (in absence of substrate): (A) [MnO4−
]
=
,
=
T
1.0 × 10−4 mol dm−3, [H2SO4]T = 1.0 mol dm−3. (B) [MnO−4
]
T = 1.0 × 10−4 mol, [H2SO4]T = 1.0 mol dm−3, [PA]T = 1.0 × 10−4 mol dm−3, (C) [MnO−4
]
T = 1.0 × 10−4 mol dm−3
[H2SO4]T = 1.0 mol dm−3, [bipy]T = 1.0 × 10−4 mol dm−3, Temp = 30 °C. (c) Absorption spectra of reaction mixture after completion of the reaction at 30 °C; [MnO4−
]
T
1.0 × 10−4 mol dm−3, [H2SO4]T = 1.0 mol dm−3, [2-butanol]T = 1.0 × 10−3 mol dm−3. (A) uncatalyzed pathway, (B) PA promoted pathway [PA]T = 1.0 × 10−3 mol dm−3, (C) bipy pro-
moted pathway [bipy]T = 1.0 × 10−3 mol dm−3
.
K1
H+
HMnO4
H3CH2C
+
MnO4
H3CH2C
K2
CH OMnO3
HMnO4
+
CH OH
H2O
H3C
H3C
H3CH2C
H3CH2C
kd
slow
+ HMnO3
C
O
CH OMnO3
H3C
H3C
fast
2 Mn2+
H+
3 H2O
+
3 MnO4
5 HMnO3
+
+
Scheme 1. Oxidation of 2-butanol by MnO−4 in absence of catalyst/promoter in aqueous acidic media.

A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
375
Mn(IV) formed during oxidation reacts with 2 mol of Mn(V) in a fast
step to give 2 mol of Mn(II) and Mn(VII), thus satisfying the stoichio-
metric observations [40]. At the same time, it is well known that, in
the reduction process of permanganate ions in strong acid medium
while the reducing species is in excess, the Mn2+ ions is the final
product.
[CPC]. In the presence of CPC, 525 nm represents the λmax of permanga-
nate–CPC ion-pair complex. The initial reaction may be represented as
Ks
MnO−4 þ CPC ⇄ MnO−4 −CPC
The following relation is used to determine the value of ion-pair
complex formation constant (Ks).
4.2. Complex formation by the promoters with permanganic acid
½MnO−4 ꢀT½CPCꢀ ½CPCꢀ
1
The experimental observations indicated that the oxidation rate was
increased with the addition of two promoters: PA and bipy. The en-
hancement in oxidation rate of 2-butanol by MnO−4 in the presence of
þ
ΔA
Δει
Ks Δει
where [MnO−4 ]T = total metal ion concentration, ΔA = the difference in
the absorbance between the complex and MnO−4 at the wave length
(525 nm) where both the uncomplexed and complexed forms of the per-
manganate absorb, Δε = the difference in absorption coefficients and l =
the path length (1 cm). The values of Ks and Δε were calculated form the
slope and the intercept. The values of Ks and Δε are 162 dm−3 mol−1 and
689 dm3 mol−1 cm−1, respectively [20]. The CMC value of CPC in the
presence of permanganate was found to be 1.0 × 10−3 mol dm−3 at
30 °C. The premicellar CPC kinetic effects as well as the micellar CPC
effects on the oxidation rate can be rationalized by considering the
distribution and/or solubilization of the reactants among the different
pseudophases present in the reaction medium. The CMC is lower than
in water (1.1 mM), suggesting that the active species of MnO−4 and/or
2-butanol interacts with a positive head group of CPC aggregates
(monomers, dimmers, trimers, etc.) and submicellar aggregates may be
formed that bring the reactants together through electrostatic interac-
tions [51]. The binding of permanganate with CPC was also confirmed
by the spectral changes observed in proton NMR spectrum of only CPC
and CPC with permanganate (Fig. S10, supplementary file). The corre-
sponding proton signals are assigned in Table S2 (supplementary file).
The permanganate interactions with CPC monomers were examined
by the chemical shifts in the CPC protons, particularly those on and
near the pyridyl ring. Addition of the CPC surfactant, above the cmc
(1.2 mM), with permanganate (1.0 × 10−3 mol dm−3) displayed
distinct upfield shifts in the proton peaks situated near the pyridyl
ring, verifying the positioning of permanganate close to the pyridyl
ring (Fig. S10). Importantly, the chemical shifts in the proton signals
of CPC from normal micelles (Fig. S10, spectrum (a) supplementary
file) and that in the presence of permanganate (Fig. S10, spectrum
(b) supplementary file) are noticeably different. This is an indication
that the CPC micelle structures get modulated by the presence of per-
manganate. Assuming the interaction of permanganate–CPC complex
above the cmc, the binding analysis of the observed (Fig. S4b supple-
mentary file) chemical shifts provided the binding constant value of
(162 dm−3 mol−1), which supports the ion–dipole interaction between
the polar permanganate and the cationic pyridyl group. On the
other hand, similar NMR measurements with permanganate and CPC
(1.2 mM, above the cmc of CPC), displayed broader proton peaks, hav-
ing very little changes in the chemical shifts of the pyridyl protons
(Fig. S10b, supplementary file) [22].
the two promoters is most probably due to the formation of MnO4−
-
promoter complex in pre-equilibrium steps. These complexes are as-
sumed here as Active-oxidant, like the Cr(VI)-promoter complexes in
Cr(VI) oxidation kinetics [27–29]. The formation of active oxidants is
supported by the proton NMR spectra of MnO−4 -promoter complex of
the corresponding promoters (Figs. S8 and S9, supplementary file).
The corresponding proton NMR signals are assigned in Table S2
(supplementary file).
The UV–vis spectrum (Fig. 4b) clearly confirms the complex forma-
tion between the oxidant and promoter. The rate constant of PA and
bipy promoted path both in the presence (kobs = 253.8 × 10−5 s−1
for 0.005 mol dm−3 of PA and 281.0 × 10−5 s−1 for 0.005 mol dm−3
of bipy) and in the absence (kobs = 8.766
×
10−5 s−1 for
0.005 mol dm−3 PA and 13.786 × 10−5 s−1 for 0.005 mol dm−3 bipy)
of CPC is far greater than the aqueous phase path (kobs = 2.75 ×
10−5) s−1 at 30 °C, suggesting a significant ligand participation in
complex formation with MnO−4 in the transition state of the oxida-
tion. Furthermore, the transition state is stabilized through coordi-
nation with HMnO4 and the nitrogen lone pair of the ligand (PA
and bipy). The cationic species of the promoters are (HA+) reactive
and major existing species in the presence of 1.0 mol dm−3 H2SO4
and it may undergo complexation with HMnO4.
It is observed that PA and 2,2′-bipyridine display characteristic fea-
ture toward the permanganate oxidation of 2-butanol. They promoted
the reaction rate. It is well established that the presence of phenyl ring
in the permanganate-promoter complex affects the decomposition of
the complex. The N-hetero-aromatic base promoters may form a com-
plex in the intermediate step probably and it is concluded by the down-
field shift of proton signals of promoters during the formation of
complex with permanganic acid. The above complexing agents have
the pyridine moiety
which now becomes electron dearth and
the aromatic protons become more deshielded. It is believed that the
catalytic activity of complexing agents such as PA and 2,2′-bipyridyl de-
pends on their ability to stabilize intermediate manganese valence
states. Quite likely, the complexing agents stabilize the end products,
manganese (II) (Fig. 4c), and hence accelerate the whole of the reaction.
This is similar to the view reported in the Cr(V) oxidation of sulfides and
cyclic ketones in the presence of PA. The reactive nucleophile Cr(VI)-
phen complex has been reported in the oxidation of anisole by chromic
acid [49,50].
4.4. Optical images
4.3. Association of MnO−4 with the CPC micelles
The optical micrographs are taken for mixtures of 10:1 concentra-
tion ratio of 2-butanol and surfactant CPC in 100× magnification in an
optical microscope (LEICA DM 1000). The images (Fig. 5) represent
the evidence of micelle formation in aqueous medium. The formation
of micelles in association with 1-butanol in aqueous medium was clear-
ly identified by the optical images found.
Interestingly, the absorbance of reaction mixture (MnO−4 + 2-buta-
nol + H2SO4) changed very rapidly in the presence of CPC (Fig. S4b, sup-
plementary file). This behavior indicates the electrostatic interaction
between the positive head group of CPC and permanganate ion or the
association of the MnO−4 into the stern layer of the CPC micelles and
results in the formation of ion-pair complex between MnO−4 and CPC.
The hyperchromic shift in the absorption spectrum observed at [CPC]
below the CMC. Surprisingly, as the [CPC] increased from 3.0 × 10−4
to 18.0 ×10−4 mol dm−3, the absorbance decreases with increasing
4.5. Scanning electron microscope (SEM) images of CPC micelle
SEM analysis was done by placing a drop of CPC solution on foil
paper. Then it was air dried followed by coating with gold, and was

376
A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
observed under Scanning Electron Microscope SEM (S530 HITACHI
SEM) instrument using IB 2 ion coater. The SEM picture (Fig. 6a) of
pure CPC looked rod-like or cylindrical particles. This also looked like a
bilayer structure. Most of the micelles appear to be bent rather than
straight cylinders. It has been known for many years that aggregates
with different morphologies can form in small molecule surfactant sys-
tems. The addition of MnO−4 changes the morphologies of the aggre-
gates from bilayers or cylinders to spheres (Fig. 6b, c). At [CPC] N CMC,
in the presence of 2-butanol the broken rock-type materials of large
size appeared in the SEM picture (Fig. 6d, e). In this concentration of
CPC in the medium, the formation of particles of varied geometries
and patterns was observed as below:
interaction occurring between CPC micelle and reactants determines
the locus of the solubilization phenomenon at the micellar interface,
in between the hydrophilic head groups and the first few carbon
atoms of the hydrophobic fragment (also commonly referred to as
the palisade layer), and finally in the core of the micelle. Therefore,
the polar –OH functional groups of 2-butanol are mainly solubilized in
the outer region of the micelle, whereas nonpolar aliphatic framework
are preferentially located in the inner portions of the micelles
(Scheme 3a–c) [53]. When concentration of the micelles was less than
that in CMC, MnO−4 ions are preferentially located at the micellar surface
due to the presence of Cl− ions. When the concentration of the surfac-
tant becomes greater than or equal to CMC, the aqueous medium is
largely populated by surfactant micelles. The MnO−4 ion approaches to-
ward the micellar head group region [54]. The electrostatic attraction of
the cationic head groups of CPC micelle surface to the nucleophilic anion
counterions leads to augmentation of the local concentration of the
nucleophile, whereas the incorporation of the substrate 2-butanol in
the micelle leads to a higher local concentration of the substrate. This
enhanced concentration of the reactants accounts for the higher rate
of reaction and thereby leading to a decrease of half-life value
(Table 1) [55–57].
4.6. High resolution transmission electron microscope (HRTEM) images of
CPC micelle
The HR-TEM investigation was done at 20 KV acceleration voltage
using lacey carbon coated Cu grid of 300 mess size in HR-TEM micro-
scope (JEOL JEM 2100). Samples were prepared by placing sample
mixture drops directly on the copper grids using a micropipette. The re-
actants including the surfactant present in the aqueous mixture were
allowed to settle. TEM pictures (Fig. 7) are presented which illustrate
the multiple morphologies of the aggregates made from the surfactant
CPC. Also, the micrograph suggests a relatively narrow size distribution
of the cylindrical micelle (Fig. 7a to c) [52] diameters, but a widely var-
iable length. With the addition of reactants, the morphology changes at
some extent may be due to the accumulation permanganate in acidic
solution at the micellar surface. Cylindrical vesicles were converted to
the spherical micelles in the presence of permanganic acid as depicted
in the images. The small particles closely associated with the large
spherical micelles (Fig. 7d to f) are permanganate species.
4.8. Probable reaction site in micelle
Micellar surfaces are water-rich and polarities of micelle–water in-
terfaces are lower than that of bulk water. The catalysis of the reaction
between 2-butanol and permanganate by CPC could be rationalized in
terms of both electrostatic and hydrophobic interactions. 2-butanol
gets incorporated hydrophobically into the palisade layer and MnO4−
may be totally present in the Stern layer due to the electrostatic interac-
tions of negatively charged species may help to stabilize MnO−4 and CPC
through complexation. It is interesting to note that the reactivity of
2-butanol toward MnO−4 is much higher than the corresponding oxida-
tion of many other alcohols in the absence and presence of cationic CPC
surfactant. This is not very surprising judging from the fact that side
chain alcohol acids play an important role: the oxidation rate increases
with hydrophobicity of the side chain as reflected in Table 1. The values
of CMC (Fig. S4, supplementary file) suggesting that the MnO−4 and
2-butanol interacts with the CPC micellar aggregates (ion pair between
positive head group of CPC and reactants) are formed. Therefore, the
environment will be less polar than in pure water.
4.7. Analysis of catalytic role of CPC
The formation of an ion-pair complex between permanganate and
the positive head group of the CPC micelles was confirmed from the ini-
tial absorbance changes at 525 nm (Fig. S2b, supplementary file). The
increase in kobs with [CPC] may be attributed to increasing solubiliza-
tion/incorporation/association of the reactant species with increase in
[CPC]. The plot of kobs versus [CPC] shows rate constant (Fig. 2) increase
monotonically nearly 70-fold with [CPC]. In a micellar system, with CPC
as the substrate, both the oxidant and the alcohol are found to be
partitioned between the aqueous and the micellar pseudophase. Thus,
the reaction may proceed according to Scheme 2.
4.9. Probable role of CPC micelle
The attractive electrostatic interaction between the positively
charged polar head groups and negatively charged reactants is in-
creased, leading to an increase in the free energy, which are advantages
in regards to the energy requirement for the redox reaction. The type of
The hydrophobic, electrostatic and hydrogen bondings are the main
factors involved in the rate enhancement of a bimolecular reaction. It
was observed that the reaction rate increases with [CPC] (Fig. 2),
which clearly demonstrates the CPC catalytic effect not only above but
Fig. 5. Different views of optical micrograph images of mixtures of 2-butanol and surfactant CPC in water: (a) and (b) 2-butanol:CPC = 10:1.

A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
377
-
(a) Only CPC (6000X)
(b) CPC+ MnO4 (2000X)
-
(c) CPC+ MnO4 (6000X)
(d) CPC+2-butanol (2000X)
(e) CPC+ 2-butanol (500X)
Fig. 6. Different views of SEM images showing micelles formed by CPC in aqueous medium. [CPC] = 1.2 × 10−3 mol dm−3 (a) SEM image of only CPC micelles in 6000× magnification;
(b) and (c) SEM image of CPC micelles showing the interaction with MnO4− in aqueous acidic medium in 2000× and 6000× magnification respectively, [MnO−4 ] = 1.0 × 10−4 mol dm−3
[H+] = 1.0 mol dm−3; (d) and (e) SEM image of CPC micelles showing the interaction with 2-butanol in 2000× and 500× magnification respectively; [2-butanol] = 1.0 × 10−4 mol dm−3
,
,
Temp = 30 °C.
also even below the CMC (i.e., micellar as well as premicellar catalyses
are observed) [39]. For 2-butanol–MnO−4 reactions, the degree of
micellar rate enhancement depends largely on the extent of reactive
counterion binding to micelles. Reactions occur between the micellar
solubilized MnO−4 and 2-butanol and the bound counterions in the junc-
tural region of palisade—Stern layers. The frequency of molecular colli-
sions increases as a consequence of the close association of the two
reacting species at the micellar interface that enhances the oxidation
rate. The promoted reaction pathway in the presence of CPC micelle
permanganate-promoter complex affects the rate due to the maximum
accumulation of reactants in the stern layer. PA and bipy get complexed
with permanganate and thus enhance the catalytic activity of CPC mi-
celle by electrostatic interaction [58] with more number of reactants
with CPC micelle. A possible arrangement (although highly schematic)
could be that as shown in Scheme 3a–c.
Bunton et al., showed that Cationic micelles of CPC, accelerate the
oxidation of thioanisole, by tetraperoxomolybdate ion, Mo(O2)24− [59].
The acceleration of rate of oxidation of alcohols by KMnO4 by Lewis
acids was carried out by Hongxia Du et al. [60]. The rate is increased
by 4 orders of magnitude in the presence of Ca2+. But in our case the
rate has been increased up to 100-fold in the presence of CPC micelle
combination with bipy promoters.
It is interesting to point out that micelles can influence the rates of
micelle-modified reactions through two different effects.
❖ The concentration or depletion of reactants in the interfacial region
has a major effect on the rates of bimolecular processes, such as the
one investigated in this work. This is called the concentration effect.
❖ Another effect depends on the transfer of substrate from water to mi-
celles, on the reaction mechanism, and on the properties of the

378
A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
Fig. 7. (a) to (c) Different views of TEM images showing micelles formed by CPC in aqueous medium, [CPC] = 1.5 × 10−3 mol dm−3. (d) to (f) Different views of TEM images of the reaction
mixture showing the interaction of MnO−4 by CPC micelles in aqueous acidic medium. [CPC]T = 1.2 × 10−3 mol dm−3, [MnO−4 ]T = 1.0 × 10−4 mol dm−3, [H+]T = 1.0 mol dm−3, Temp = 30 °C.
interfacial region, such as local charge, polarity, and water content.
The kinetic micellar effects observed in bimolecular processes are
mainly due to the concentration of reactants in the small volume of
the micelles.
5. Conclusion
Our present civilization is highly cursed with ecological pollution,
which threatens the very existence of life. Potassium permanganate
has long been a suitable oxidant to neutralize some of these pollutants
and many studies are progressing in this direction. Permanganate oxi-
dation has gained momentum since more than a century ago. Recently,
a large number of studies have been carried out without organic solvent
in aqueous micellar medium.
❖ In the reaction studied, the difference between kobs(w) and kobs(m) in
the presence of 5.0 × 10−3 mol dm−3 bipy is 278.25 × 10−5 s−1
and accounts for the micellar medium effects in the micellar solutions
assuming that no morphological transition occurs [61].
The experimental data in Table 1 describes the half life (t1/2) values
of the 2-butanol oxidation reaction by MnO−4 that gradually decreases
upon addition of PA or bipy and CPC micellar catalyst. The decrease in
The key facts are:
✓ (1) The reaction proceeds fast in the presence of PA/bipy promoter
in CPC surfactant than in aqueous phase.
t1/2 value (0.0685 h) becomes more severe when the combined effect
of CPC micellar catalyst and bipy promoter has come into play. The over-
all analysis of our experimental results compared to the literature works
are presented in Table 2. The pseudo-first-order rate constants of the
oxidation reaction between MnO−4 and 2-butanol vary in magnitude
✓ (2) Both the reactants (MnO−4 and 2-butanol) proceed toward the
cationic head group of CPC micelle.
✓ (3) The increase in the rate of reaction caused by the presence of
micelle is due to the increase in the number of collisions between re-
actant molecules and stabilizes the intermediate by the opposite
charge of cationic surfactant. SEM, TEM and NMR experiments are
helpful to determine the interaction of CPC micelle with MnO4−
and 2-butanol.
in the order of k(CPC) N k(Aqueous), k(bipy) N k(PA) and k(bipy)(CPC)
N
k(PA)(CPC). The micellar catalysis by CPC in association with bipy promot-
er exhibited the remarkable ~100-fold rate enhancements (Table 1) as
compared to that of uncatalyzed reaction.
kW
HMnO4
+
2-butanol
2-butanone
W
W
W
KS
KO
kM
2-butanol
+
HMnO4
2-butanone
M
M
M
Scheme 2. Partitioning of the oxidant and substrate between the aqueous and micellar pseudo phase. W = aqueous medium, M = CPC micellar medium, S = substrate (2-butanol), O =
oxidant.

A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
379
(a)
(b)
H3CH2C
H3C
H3CH2C
H3CH2C
H3C
H3CH2C
H3C
N
COOH
+
CH OH
C
O
HMnO4
+
CH OH
HMnO4
C
O
H3C
H+
H+
Gouy-Chapman
layer
Gouy-Chapman
layer
MnO4
Stern layer
MnO4
N
COOH
HMnO4
H+
+
+
HMnO4
COOH
+
+
N
+
N
N
N
+
N
N
+
H+
+
N
+
N
HMnO4
N
+
MnO4
HMnO4
N
MnO4
H+
MnO4
N
+
N
H+
+
N
O4
n
+
M
N
N
COOH
+
N
+ N
Palisade layer
+
N
Palisade layer
+
N
N +
N
+
N
+
N
MnO4
N
+
COOH
H+
N
Mn
O4
+
N
H+
+
N
+
MnO4
N
+
H+
N
N
+
MnO4
N
+
N
COOH
+
N
N
+
HMnO4
N
+
N
+
HMnO4
H+
+
HMnO4
HMnO4
Stern layer
MnO4
H+
MnO4
N
COOH
H+
HMnO4
HMnO4
C2H5 group
H
(c)
OH group
+N
H3CH2C
H3CH2C
N
N
Cl- ion
CPC monomer
+
CH OH
HMnO4
C
O
CH3 group
H3C
H3C
H+
2-butanol
Gouy-Chapman
layer
MnO4
N
N
HMnO4
+
+
N
+
N
N
H+
+
N
+
HMnO4
N
MnO4
H+
MnO4
N
N
+
N
+
N
N
N
+ N
Palisade layer
N +
+ N
N
N
N
+
MnO4
N
+
H+
HMnO4
N
+
MnO4
N
N
N
+
N
N
+
H+
+
HMnO4
Stern layer
MnO4
N
N
H+
HMnO4
Scheme 3. (a) Schematic model showing probable reaction site for the cationic (CPC) micellar mediated oxidation reaction between (a) [MnO−4 ], 2-butanol and proton; (b) [MnO4−], picolinic
acid, 2-butanol and proton (c) [MnO−4 ], 2,2′-bipyridine, 2-butanol and proton.
The proper choice of a surfactant can lead to rate increases of
5- to 1000-fold compared to the same reaction in the absence of
a surfactant. Bipyridine exhibited 100-fold rate enhancement of
the normal MnO−4 oxidation of 2-butanol at room temperature.
In our study the combination of CPC and bipy was the most
excellent micellar catalyst and promoter for the MnO4−
oxidation of 2-butanol to 2-butanone in acidic medium at room
temperature.

380
A. Ghosh et al. / Journal of Molecular Liquids 198 (2014) 369–380
Table 2
A comparative result for oxidation of 2-butanol using different oxidant and catalyst.
Entry Catalyst
Ref.
Conditions
Rate constant (kobs)
1.
2.
–
[62]
[63]
[2-Butanol]T = 3 × 10−3 mol dm−3, oxidant = FeO24−, pH = 9.70, T = 25 °C.
1.40 × 10−2 dm3 mol−1 s−1
10.47 × 10−4 s−1
[Cr(III)] = 20 × 10−4 mol dm−3
[2-Butanol]T = 4.0 × 10−2 mol dm−3, [Ce(IV)] = 4.0 × 10−3 mol dm−3
,
[Cr(III)] = 20 × 10−4 mol dm−3, [H2SO4] = 6.0 × 10−2 mol dm−3
,
[HClO4] = 2.0 mol dm−3, μ = 2.1 mol dm−3, T = 30 °C.
3.
[CHAPS] = 8 × 10−3 mol dm−3
[Ir(III)] = 2 × 10−6 mol dm−3
,
[18]
[2-Butanol]T = 2.0 × 10−3 mol dm−3, [Ce(IV)] = 2 × 10−4 mol dm−3
,
1.55 × 10−2 s−1
[CHAPS] = 8 × 10−3 mol dm−3, [Ir(III)] = 2 × 10−6 mol dm−3
,
[H2SO4] = 0.5 mol dm−3, μ = [H2SO4–Na2SO4] = 2.0 mol dm−3, T = 30 °C.
4.
5.
[Ru(III)] = 4.8 × 10−6 mol dm−3 [64]
[Ru(VI)] = 4.8 × 10−6 mol dm−3 [65]
[2-Butanol]T = 2.0 × 10−2 mol dm−3, [K3Fe(CN)6] = 3.0 × 10−3 mol dm−3
[Ru(III)] = 4.8 × 10−6 mol dm−3, [NaOH] = 2.0 × 10−2 mol dm−3, T = 30 °C.
,
7.29 × 10−5 mol dm−3 min−1
13.75 × 10−5 mol dm−3 min−1
[2-Butanol]T = 0.80 mol dm−3, [K3Fe(CN)6] = 2.0 × 10−3 mol dm−3
,
[Ru(III)] = 4.8 × 10−6 mol dm−3, [NaOH] = 0.2 mol dm−3
,
μ = 0.5 mol dm−3 T = 30 °C.
6.
7.
8.
–
–
[66]
[67]
[2-Butanol]T = 0.50 mol dm−3, [CTADC] = 1.89 × 10−4 mol dm−3
medium = DCM, [acetic acid] = 3.24 mol dm−3, T = 30 °C.
,
3.8 × 10−3 s−1
6.11 × 10−5 s−1
2.81 × 10−3 s−1
[2-Butanol]T = 2.0 × 10−2 mol dm−3, [K2S2O8] = 2.0 × 10−2 mol dm−3, pH = 8,
T = 60 °C.
[CPC] = 1.2 × 10−3 mol dm−3and Present [2-Butanol]T = 1.0 × 10−3 mol dm−3, [MnO4−] = 1.0 × 10−4 mol dm−3
,
[bipy] = 5.0 × 10−3 mol dm−3
work
[CPC] = 1.2 × 10−3 mol dm−3, [bipy] = 5.0 × 10−3 mol dm−3
[H2SO4] = 1.0 mol dm−3, T = 30 °C.
,
Here, CTADC = Cetyltrimethylammonium dichromate, DCM = Dichloromethane.
[28] K. Mukherjee, R. Saha, A. Ghosh, S.K. Ghosh, B. Saha, Spectrochim. Acta A 101 (2013)
294–305.
Acknowledgments
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Thanks to UGC ((F. No. 33-495/2007 (SR) dated 28.02.2008), New
Delhi and CSIR 01 (2463)/11/EMR-II, Dated 16-05-2011), New Delhi
for providing financial help in the form of a project and fellowship.
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